Phosphorous (P) in ferro manganese used as an alloy of iron in steelmaking is a factor deteriorating the quality of products steel, for example, a cause of high temperature brittleness. Accordingly, dephosphorization removing phosphorous (P) from molten ferro manganese, i.e., ferro manganese melt-pool is generally conducted.
In a typical dephosphorization process for producing ferro manganese, melt-pool is poured into a ladle and an impeller is submerged into the melt-pool to stir the melt-pool. Herein, a general impeller 20 is provided with wings, i.e., blades at a lower side of a stirring shaft as disclosed in Korean Patent Publication No. 2011-0065965. Again describing the general impeller with reference to FIG. 2, the impeller includes an impeller body 21 extending in a longitudinal direction thereof, a plurality of blades 22 connected to a circumferential surface of a lower portion of the impeller body 21, an blowing nozzle 23 configured to pass through each of the plurality of blades 22, a supply tube 24 configured to pass through inner centers of the impeller body 21 and the blades 22 and to supply a dephosphorization agent and gas, and a flange 25 connected to an upper end of the impeller body 21. The flange 25 is connected to a driving unit (not shown) providing rotational power.
A stirring flow by an operation of the impeller 20 will be described below in brief. As shown in FIG. 2, a stirring flow (arrow of solid line) generated in an inner wall direction by the rotation of the blades 22 collides with an inner wall of the ladle 10, and then is divided and flows into up and down directions along the inner wall of the ladle 10. Then, a flow in which the dephosphorization agent and gas sprayed from the blowing nozzle 23 ascends along outer circumferential surfaces of the blades 22 and the impeller body 21 collides with a flow in which the dephosphorization agent and gas collide with the inner wall of the ladle 10 by the rotation of the blades 22, then ascend, and again descend. Also, the flow in which the dephosphorization agent and gas ascend along the outer circumferential surfaces of the blades 22 and the impeller body 21 and then again fall along the inner wall of the ladle 10 collides with the stirring flow which is generated by the rotation of the blades 22 and ascends along the inner wall of the ladle 10. A stirring force is cancelled by the collision of these flows, which becomes a factor to reduce the rate of reaction between the melt-pool and the dephosphorization agent and to thus reduce the dephosphorization rate.
Meanwhile, as a method of controlling a phosphorous component in the melt-pool, there is a method which removes phosphorous (P) in the melt-pool in the form of phosphorous oxide (Ba3(PO4)2 or the like) through oxidation dephosphorization. The dephosphorization agent for controlling the phosphorous component in the melt-pool may include BaCO3, BaO, BaF2, BaCl2, CaO, CaF2, Na2CO3, and Li2CO3, and may be in the form of flux.
Among these, since the Ca-based materials have low dephosphorization efficiency and the Na- and Li-based materials have high vapor pressure, a rephosphorization phenomenon is generated. Since it is known that the higher the alkalinity, the higher the dephosphorization performance of the dephosphorization agent as dephosphorization flux, Ba-based compounds (BaCO3, BaO, etc.) that have high alkalinity and do not have high vapor pressure have been mainly used and developed. However, when the Ba-based compounds are used as the dephosphorization agents, the high melting point thereof allows a phosphorous component to be obtained in the form of solid, so that there is a problem that the dephosphorization efficiency is reduced. Accordingly, in order to address such an issue, methods of adding BaCl2, BaF2, NaF2 or the like have been developed. In the case of BaCl2, slag on the ferro manganese is scattered by vaporization of chlorine (Cl) group having strong volatility and flies away, and facility corrosion may be caused by volatilization of Cl group. Also, since BaF2 is very expensive, BaF2 is difficult to use in terms of establishing an economical production process. Further, NaF2 is volatilized to fly away with the course of treatment process time, and thus the concentration thereof is lowered. Eventually, only a decrease of the melting point may be expected by the F effect, and in order to overcome this issue, it is necessary to increase the content of NaF2.
When the slag has a very high melting point, in order to obtain the flux effect, there is a method of producing a Ba-based dephosphorization agent in liquid form for use thereof in addition to a method of adding elements other than Ba-based elements (Application No. 2011-0093754). When the dephosphorization agent is used in liquid form, a temperature drop due to the adding of a solid dephosphorization agent with a relatively low temperature may be suppressed, and skull generation due to the solidification phenomenon may be prevented to increase the dephosphorization effect, which leads to the improvement of recovery of ferro manganese after the dephosphorization. Furthermore, there is an advantage that a mixing amount of raw materials (BaCl2, BaF2, NaF, etc.) considered as the flux may be reduced or any of the raw materials may be excluded in accordance with the liquefaction temperature of the dephosphorization agent.
However, in the aforementioned method of using the liquefied and melted dephosphorization agent, since a liquefaction method is a method of heating a dephosphorization agent to a temperature higher than a melting point thereof and liquefying the dephosphorization agent, although the dephosphorization agent is liquefied at a temperature higher than the melting point thereof to be used when the melting point of the dephosphorization agent used is very high, a difference between the melting point and the liquefied temperature is decreased, so that an applicable range is narrow. Also, generally, when a difference between the melting point of dephosphorization agent and the liquefied temperature is decreased due to a high melting point thereof, fluidity of the dephosphorization agent is very low, so that it is very difficult to control in adding a liquid dephosphorization agent.
Further, in order to maintain alkalinity of dephosphorization slag at a high level in a dephosphorization process using a Ba-based dephosphorization agent, a BaO content functions as a major criterion. However, in the case of BaO, dephosphorization slag can be maintained in a state of high alkalinity, but it is difficult to use BaO by itself as a dephosphorization agent in a real process. BaO can be produced through a calcination reaction of BaCO3, but the produced BaO is easily hydrated due to very high reactivity with moisture. In addition, when BaO is converted into a hydrate such as Ba(OH)2 or the like, the Ba(OH)2 reacts with CO2 in the air to be converted into BaCO3, so that there are troubles such as storage. Therefore, typically, when a Ba-based dephosphorization agent is used, BaCO3 is used as a main raw material. When BaCO3 is used, a CO2 gas is generated while a calcination reaction is performed in a high temperature ferro manganese melt-pool, so that the generated CO2 gas functions to massively supply oxygen, and BaO generated through the calcination reaction is contained in slag to maintain alkalinity of the slag at a high level. However, the CO2 gas generated through the calcination reaction of BaCO3 oxidizes Mn in the ferro manganese melt-pool, and thus the content of Mn oxide in the slag is increased to lower the alkalinity of the slag. Also, as a dephosphorization refining process continues, since the melt-pool is exposed to the air by the introduction of the dephosphorization agent and the continuation of process time, a temperature thereof is dropped, and an oxidizing of Mn is promoted, so that the dephosphorization efficiency of the dephosphorization agent is lowered.
When a solid dephosphorization agent, for example, a BaCO3—NaF-based dephosphorization agent is used at the beginning, an initial melting point is high and BaCO3 is calcinated through a high temperature refining reaction to increase the amount of BaO. Although a eutectic composition of BaO—BaCO3 is made, it is difficult to achieve liquefaction due to component imbalance. Also, during a refining process, since an oxidized MnO component is contained to cause component imbalance, solidification or skull takes places and as a result, it is more difficult to achieve liquefaction.